Natural gas hydrates (NGH) are a kind of cage-like crystalline compound formed from natural gas and water under a low temperature and high pressure. With a snow and ice-like appearance, NGHs, commonly known as “combustible ice”, will burn immediately in case of a fire. The natural gas in the natural gas hydrates found in nature is mainly composed of methane (>90%). Under the normal temperature and pressure, 1 m3 of natural gas hydrate decomposes and releases about 160 m3 of natural gas, so natural gas hydrates have an extremely high energy density. The natural gas hydrates found in nature mainly exists in the sediments of the continental shelf of the oceans and the continental tundra. In 1964, scientists discovered natural gas hydrates in the Siberian tundra for the first time, and soon also discovered natural gas hydrates in the seabed sediments in the Black Sea. By the 1990s, scholars in the industry agreed that the energy stored in the global natural gas hydrates exceeded the total energies stored in all oil, coal and natural gas. Over the past 20 years, a plurality of programs including deep sea drilling programs (DSDP), the ocean drilling programs (ODP) and integrated ocean drilling programs (IODP) have been launched worldwide to investigate the mineral resources of natural gas hydrates. At present, the total volume of the global natural gas hydrates is estimated to be about 1015˜1018 standard cubic meters. Therefore, natural gas hydrates are considered as the energy source with the most potential for replacing oil and natural gas in the 21st century. A survey on resources shows that China's South China Sea, the continental slope of the East China Sea—Okinawa, and the tundra of the Qinghai-Tibet Plateau are all reserves of natural gas hydrates. Therefore, carrying out studies on the effective, rapid and economical method for exploiting natural gas hydrates to provide an experimental basis and evidence for large-scale exploitation of natural gas hydrates is an effective way to alleviate the increasing pressure on energies.
The exploitation technology for natural gas hydrates is one of the key links to realize the development and utilization of natural gas hydrate resources. Unlike conventional fossil fuels, natural gas hydrates are present in porous media in solid form. The basic idea of the exploitation is to change the temperature—pressure environments, or the phase equilibrium conditions of the hydrates, under which natural gas hydrates can exist stably so as to make the solid hydrates in the reservoir in situ decompose into natural gas and water, and then the natural gas can be exploited. Accordingly, scientists have proposed several conventional exploiting technologies, such as the pressure reduction method, the heat stimulation method and the chemical reagent method. Due to the complicated geological environment and the diversified forms of the mineral reserves of the hydrates, as well as the complicated phase change process and multi-phase seepage process of the multi-phase system composed of the natural gas-water-sediment-hydrate-ice, the change of the skeleton of the porous media accompanying the hydrate decomposition during the exploitation of hydrates is one of the largest problems encountered during the exploitation of hydrates currently. The original geological characteristics of the mineral reserves of the hydrates will change greatly (for example, the permeability, the porosity, the mechanical properties and the pore pressure will all change drastically), as natural gas hydrates reserved in solid form change into flowing water and gas, thus resulting in the deformation of the porous media and the gas-solid-liquid three-phase mixed flow field, which may eventually lead to ground deformation. Therefore, carrying out studies on the influence of the sediment yield behavior on the deformation of porous media in the decomposition of natural gas hydrates plays an important role in the successful completion of the hydrate production technology and the safety thereof.
Currently, the studies on the relatively advanced exploitation of natural gas hydrates around the world are focused on the effect of different exploiting methods for the phase-change decomposition of the hydrates and the consuming transfer of the heat in the decomposition of the hydrates. For a real condition, the understanding of the complicated mechanism of the phase-change seepage in the decomposition of the hydrates is still in a vague state. In almost all simulated experiments of hydrate exploitation, the relationship between sediment yield behavior and the deformation of porous media during the exploitation of hydrates has been neglected. In the prior simulated experiments, the porous media (particle diameter >100 μm) composed of large particles were used to make the skeleton of the porous media unchangeable during the decomposition of the hydrates, but the porous media with actual particle diameters in the mineral reserves of the hydrates are composed of particles with sizes ranging from 0.01 μm (micro-particles) to 500 μm (large particles) together, and sediment yield behavior and deformation are unavoidable during the exploitation of hydrates. Particularly, the radial deformation of the porous media around the well may be the main factor resulting in the collapsing of the well wall and sediment yield behavior. At present, there are no effective experimental methods for measuring the radial deformation caused by hydrate decomposition. Currently, the experimental devices of natural gas hydrates exploitation are becoming more accurate and are developing in line with the actual modes of the outdoors; however, the implementation of specific projects is still faced with great challenges. The research achievements of the existing laboratory devices cannot fully meet the needs for technologies providing safe and economic field exploitation of the hydrates, and there is a need for further research and development of advanced experimental equipment and platforms which enable a more accurate inversion of the actual changes during the exploitation of hydrate reserves in the seabed and the operating situations of the exploiting equipment so as to lay a solid foundation for the realization of safe and reliable exploitation.